Which Caster should I focus on?

Probably Da Vinci, as it's generally better to prioritize SSRs.

Nitocris for farming, Da Vinci for general use.

They're both AoE casters, and I think Da Vinci is stronger. Nitocris has the instant death niche which is pretty weak, but a better np charge skill. Da Vinci has the benefit of the extra stats from a higher level cap and an np damage steroid, both of which i think are more desirable than anything Nitocris has.

People underestimate Nitocris far too much there is a reason appmedia has her tied with Mash as the strongest 4*. Well the major downside of her is you really need 10/8 on her first 2 skills for her to function properly but once you have that she is great.

If you need something for bossing Da Vinci is better though.

Nitocris is an amazing farming servant and almost unparalled at that because of the high instant death gimmick. Her being a 4 star compared to Da Vinci just makes her better thanks to her lower ascension and skill costs. Da Vinci has limited use in the game especially without a big investment to her and her interlude that buffs her NP being almost 2 years away.
TLDR: Da Vinci if you have materials to burn and really like her(Whats your friend code if you do cause I love Da Vinci)
OR
Use Nitocris since she can farm every daily even without class advantage < HIGHLY Recommended option

I don't feel like doing the math because its midnight and I need to be up in 7 hours but doesn't the fact that OP's nitocris is np3 play a factor? Leo's atk is only 1500 more than nito's. Does the 30% np damage from Leo really compensate for the...what was it...200% difference with nito's np 3? Therwa also the fact that with the right set up nitocris can throw out 2 np in two turns